1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotographic technology and, more particularly, relates to a charging device for electrically charging a photosensitive drum.
2. Description of the Related Art
FIG. 7 is a schematic diagram of a conventional tandem-type color image forming apparatus (printer) 100, and FIG. 8 is a diagram showing the configuration of a portion of the image forming apparatus 100 of FIG. 7 around charging rollers 2a, 2b, 2c and 2d and photosensitive drums 1a, 1b, 1c and 1d. The image forming apparatus 100 incorporates within a main apparatus body thereof four image forming units Pa, Pb, Pc and Pd which are arranged in this order from an upstream side to a downstream side along a sheet feeding direction (right to left as depicted in FIG. 7) of a sheet P. Provided for forming images in four different colors (i.e., magenta, cyan, yellow and black), these image forming units Pa to Pd individually perform steps of charging, exposure, development and image transfer to sequentially produce magenta, cyan, yellow and black images.
The image forming units Pa, Pb, Pc and Pd include the photosensitive drums (image carrying members) 1a, 1b, 1c and 1d which carry visible images (toner images) of the aforementioned four colors, respectively. Driven by a belt driving source (not shown), a transport belt 50 transports a sheet P while turning counterclockwise as shown by an arrow in FIG. 7 in contact relation with the individual image forming units Pa to Pd. The toner images formed on the individual photosensitive drums 1a to 1d are successively transferred onto the sheet P as the same is conveyed by the transport belt 50. The toner images so transferred on the sheet P are fixed thereto in a fixing unit 7, and then the sheet P is discharged to the outside of the apparatus main body. The photosensitive drums 1a to 1d are rotated clockwise in FIG. 7 while the image forming process is performed.
Sheets P to which toner images are transferred are stored in a paper cassette 16 provided at the bottom of the main apparatus body. The sheet P is fed to the image forming units Pa to Pd by a sheet feeding roller 12a and registration rollers 12b. The transport belt 50 is made of a dielectric plastic sheet of which flange portions are lapped and spliced to form an endless belt or formed into a seamless belt. On the upstream side of a driven roller 11, there is provided a cleaning blade 19 for removing residual toners adhering to the transport belt 50.
Next, the image forming units Pa to Pd are described. In spaces around and above the rotatably mounted photosensitive drums 1a, 1b, 1c, 1d, there are provided the charging rollers 2a, 2b, 2c, 2d for electrically charging the photosensitive drums 1a, 1b, 1c, 1d, light-emitting diode (LED) heads 4a, 4b, 4c, 4d for exposing the photosensitive drums 1a, 1b, 1c, 1d based on image information, development units 3a, 3b, 3c, 3d for forming toner images on the photosensitive drums 1a, 1b, 1c, 1d, and cleaning units 5a, 5b, 5c, 5d for removing developers (toners) left unused on the photosensitive drums 1a, 1b, 1c, 1d, respectively.
When a user enters an instruction for starting an image forming task, the image forming apparatus 100 first causes the charging rollers 2a to 2d to uniformly charge surfaces of the photosensitive drums 1a to 1d. The charging rollers 2a to 2d are chargers of a type which applies a voltage to uniformly charge the drum surfaces under conditions where the chargers are held in contact with the surfaces of the photosensitive drums 1a to 1d. Often used as these chargers are, in particular, charging rollers which are made of high-resistance rubber rollers.
Subsequently, the LED heads 4a to 4d emit light to form electrostatic latent images on the individual photosensitive drums 1a to 1d according to image signals. The development units 3a to 3d include development rollers (developer carrying members) mounted face to face with the photosensitive drums 1a to 1d and are filled with specific amounts of magenta, cyan, yellow and black toners by toner filling units (not shown) for filling the relevant color toners. The color toners are supplied to the photosensitive drums 1a to 1d by the development rollers of the development units 3a to 3d and electrostatically adhere thereto, thereby forming color toner images having the same patterns as the electrostatic latent images formed when exposed by the LED heads 4a to 4d, respectively.
After an electric field to the transport belt 50 is created by applying a specific transfer voltage thereto, the magenta, cyan, yellow and black toner images on the photosensitive drums 1a to 1d are sequentially transferred to the sheet P, conveyed by the transport belt 50, by means of respective transfer rollers 6a, 6b, 6c, 6d. These four color images are transferred to the sheet P with a predefined positional relationship for forming a desired full-color image. Then, in preparation of a succeeding electrostatic latent image forming task, residual toners left unused on the photosensitive drums 1a to 1d are scraped off by cleaning blades 5aa, 5ba, 5ca, 5da provided in the respective cleaning units 5a to 5d as shown in FIG. 8.
The transport belt 50 is tensioned between a driving roller 10 and the driven roller 11 and turns counterclockwise as illustrated when the driving roller 10 is driven to rotate by a driving motor (not shown). As the transport belt 50 turns counterclockwise, the sheet P is fed with correct timing by the registration rollers 12b to the image forming units Pa to Pd, in which the individual color toner images are sequentially transferred to the sheet P at nips formed between the individual photosensitive drums 1a to 1d and the transport belt 50 to produce the full-color toner image on the sheet P. The sheet P carrying the full-color toner image is then fed into the fixing unit 7.
As the sheet P fed into the fixing unit 7 passes through a nip (fixing nip) formed between a pair of fixing rollers 13, the toner image is fixed to a surface of the sheet P due to application of heat and pressure by the fixing rollers 13 and, at this point, the desired full-color image is completed. The sheet P carrying the completed full-color image is discharged onto a sheet delivery tray 17 by discharge rollers 15. When producing a monochrome image, on the other hand, the image forming apparatus 100 carries out an image forming task by operating the image forming unit Pd alone in essentially the same way as discussed above.
In the image forming apparatus employing the electrophotographic technology, the magenta, cyan, yellow and black toner images formed on the surfaces of the photosensitive drums 1a to 1d are sequentially transferred to the sheet (recording medium) P as the recording medium goes into contact with the individual photosensitive drums 1a to 1d carrying the color toner images. In full-color mode for forming a full-color image, all of the photosensitive drums 1a to 1d are electrically charged by the respective charging rollers 2a to 2d to produce magenta, cyan, yellow and black toner images, whereas in monochrome mode for forming a monochrome image, only the photosensitive drum 1d is electrically charged by the charging roller 2d to produce a black toner image.
Commercially, however, the image forming apparatus is operated more often in the monochrome mode for forming (printing) monochrome images than in the full-color mode for forming (printing) full-color images, so that the charging roller 2d for black is used more often than the other charging rollers 2a-2c for the three different colors. As a result, the charging roller 2d wears out more quickly compared to the other charging rollers 2a-2c. Since the life of the charging roller 2d becomes shorter than those of the charging rollers 2a-2c, the charging roller 2d needs to be replaced earlier than the other charging rollers 2a to 2c. 
Under such circumstances, after replacing the charging roller 2d, it would be necessary to replace one or more of the charging rollers 2a-2c depending on the degree of wear of these charging rollers 2a-2c. Therefore, compared to a case where all of the charging rollers 2a to 2d for black and individual colors are replaced at once, a practice of replacing the charging rollers 2a-2c results in almost twice as long down time of the image forming apparatus, and this imposes considerable work load to the user and service personnel.
To cope with the aforementioned problem of the conventional color image forming apparatus, various methods for lengthening the useful life of the charging roller for black have been proposed. For example, Japanese Patent No. 3587094 describes a tandem-type color image forming apparatus having a plurality of photosensitive drums, in which a charging device accompanying a photosensitive drum on which a black toner image is formed is a non-contact-type charger and charging devices accompanying photosensitive drums on which color toner images (other than the black toner image) are formed are contact-type charging rollers. This arrangement is intended to reduce wear of the photosensitive drums for black toner image due to contact with the charging roller, making it possible to use the photosensitive drums and the corresponding charging devices for the individual colors for the same period of time and eventually reduce running cost.
On the other hand, Japanese Unexamined Patent Publication No. 2005-346028 describes an image forming apparatus in which charging devices employing charging rollers which are held out of contact with image carrying members. Specifically, each of the charging rollers includes a core metal, a charging member made of a conductive resin material and integrally mounted around the core metal, and a gap spacing member made of an insulating resin material and mounted on both ends of the charging member. This arrangement is intended to maintain a highly accurate charging gap, reduce changes and deviation of the charging gap as well as smearing of each charging roller, prevent the occurrence of abnormal images, and achieve a cost reduction and enhanced durability.
The arrangement of Japanese Patent No. 3587094 however employs a corotron charger for black toner image forming, and the use of the corotron (or scorotron) charger can potentially result in an increase in the amount of ozone emission. In particular, if the apparatus is of a type using an amorphous silicon (a-Si) type photosensitive drum, the amount of ozone emission increases even more as it is necessary to supply an enormous amount of charging current to the corotron charger.
Additionally, in a case where the charger and the corresponding photosensitive drum are combined into a single unit, it would be necessary to replace the complete unit when the useful life of the charger is over. This makes it necessary to design the charger to have a longer useful life. Especially when the charger is used to electrically charge an a-Si type photosensitive drum, the useful life of the charger tends to become shorter compared to a case where the charger is used to charge an organic photosensitive drum, so that this arrangement would have greater influence on the useful life of the charger.
For reasons stated above, it is necessary to use a charging roller, as the charger for black toner image forming, which emits a smaller amount of ozone and to lengthen the useful life of the charger. Also, if an external toner additive or the like which are not scraped off by a cleaning blade for collecting residual toners left unused on the photosensitive drums adheres to the charging rollers, the useful life of the charging rollers is likely to become shorter. Thus, in order to ensure that the charging rollers for the four colors (including black) are to be replaced at the same time, it is necessary to reduce the amount of the external toner additive or the like adhering to a surface of the charging roller for black.
Furthermore, some users may use a particular color more often than the other colors. Accordingly, the charging roller for such specific color is required to have a longer life than the other charging rollers, meeting specific printing requirements and applications of the individual users.
On the other hand, while the Japanese Unexamined Patent Publication No. 2005-346028 discloses that the charging rollers electrically charge the photosensitive drums without contact, the Publication does not disclose or suggest any specific arrangement for switching the charging rollers between contact position with the corresponding photosensitive drums and non-contact position with the corresponding photosensitive drums.